Biondi and Sava (1999) introduce a method of migration velocity analysis using wave-equation techniques (WEMVA), which aims at improving the quality of migrated images, mainly by correcting moveout inaccuracies of specular energy. The slowness model is estimated by finding the slowness perturbation which explains the difference between the migrated image using the reference model and an externally-defined target image Sava and Symes (2002).
The moveout information given by the specular energy is not the only information contained by an image migrated with the incorrect slowness. Non-specular diffracted energy is present in the image and clearly indicates slowness inaccuracies. Since a difference between an inaccurate image and a perfectly focused target image contains both specular and non-specular energy, WEMVA is naturally able to derive velocity updates based on both these types of information. In contrast, traveltime-based MVA methods cannot easily deal with the diffraction energy, and are most of the time concerned with moveout analysis.
In this paper, we examine the resolving power of the unfocused diffraction energy present in migrated images. The target applications of these techniques are in areas of complicated geology in which diffractions are abundant and can be clearly identified and isolated. Examples include highly fractured reservoirs, carbonate reservoirs, rough salt bodies and reservoirs with complicated stratigraphic features.
Of particular interest is the case of salt bodies. Diffractions can help estimate more accurate velocities at top of salt, particularly in the cases of rough salt bodies. Moreover, diffraction energy may be the most sensitive velocity information we have under salt, since most of the reflected energy we record at the surface has only a narrow range of angles of incidence at the reflector, rendering the analysis of moveout useless.